Context Locals



Sooner or later you have some things you want to have in every single view or helper function or whatever. In PHP the way to go are global variables. However that is not possible in WSGI applications without a major drawback: As soon as you operate on the global namespace your application is not thread safe any longer.

The python standard library comes with a utility called "thread locals". A thread local is an global object where you can put stuff on and get back later in a thread safe way. That means whenver you set or get an object to / from a thread local object the thread local object checks in which thread you are and delivers the correct value.

This however has a few disadvantages. For example beside threads there are other ways to handle concurrency in Python. A very popular approach are greenlets. And also if every request gets its own thread is not guaranteed in WSGI. It could be that a request is reusing a thread from before and data is left in the thread local object.

werkzeug.local is filling this gap.


Here a simple example how you can use werkzeug.local:

from werkzeug import Local, LocalManager

local = Local()
local_manager = LocalManager([local])

def application(environ, start_response):
    local.request = request = Request(environ)

application = local_manager.make_middleware(application)

Now what this code does is binding request to local.request. Every other piece of code executed after this assignment in the same context can safely access local.request and will get the same request object. The make_middleware method on the local manager ensures that everything is cleaned up after the request.

The same context means the same greenlet (if you're using greenlets) in the same thread and same process.

If a request object is not yet set on the local object and you try to access it you will get an AttributeError. You can use getattr to avoid that:

def get_request():
    return getattr(local, 'request', None)

This will try to get the request or return None if the request is not (yet?) available.

Manager Objects

Local objects cannot manage themselves, for that you need a local manager. You can pass a local manager multiple locals or add additionals later by appending them to manager.locals and everytime the manager cleans up it will clean up all the data left in the locals for this context.

The following methods exist:

Manually clean up the data in the locals for this context. Call this at the end of the request or use make_middleware().
Encapsulate the application and call cleanup() at the end of a request.

Like make_middleware but works as decorator. The main difference is that the returned object will have the original docstring etc.

def application(environ, start_response):
Return the context identifier the local objects use internally for this context. You cannot override this method to change the behavior but use it to link other context local objects (such as SQLAlchemy's scoped sessions) to the werkzeug locals.


Werkzeug can also create proxy objects for you. A proxy object forwards nearly all operations to an object bound to an object on a local. This is for example useful if you want a global request object:

from werkzeug import Local
local = Local()
request = local('request')
user = local('user')

Whenever you access anything on user it will forward the operation to the object local.user. This affects all operations except of any sort of assignment.

Keep in mind that the repr is also forwarded so if you want to find out if you are dealing with a proxy you can do an isinstance check:

>>> from werkzeug import LocalProxy
>>> isinstance(request, LocalProxy)

You can also create proxy objects by hand:

from werkzeug import Local, LocalProxy
local = Local()
request = LocalProxy(local, 'request')